1. Field of the Invention
The present invention is accomplished for elimination of the above-mentioned problems. The object of the present invention is to offer an Al-coated steel sheet, and preferably a fuel tank made from an Al-coated steel sheet, which satisfies requisitions for use as a fuel tank superior of corrosion resistance and durability.
2. Prior Art
A fuel tank 1 for an automobile or the like has been conventionally manufactured by the following manner: An upper half 2 and a lower half 3 are independently prepared from steel sheets by press-working, and then washed with an alkali liquid. Halves 2 and 3 are assembled to a tank shape, joined together by resistance-welding such as seam-welding and spot-welding, and then coated with an anti-corrosion paint at their whole external surfaces, as shown in FIG. 2.
The upper half 2 and the lower half 3 have sinks 4 of the same openings and shapes. Flanges 5 are formed at the whole periphery of the sinks 4. These halves 2 and 3 are joined to a unitary body by putting the flange 5 of the upper half 2 on the flange 5 of the lower half and welding together. Since a material is worked to the fuel tank with heavy duty, it shall be good of press-workability, resistance-weldability and post-coat corrosion-resistance. In the state that such the fuel tank is installed in an automobile, its inner surface is exposed to a corrosive atmosphere containing a fuel. If the inner surface is corroded by the fuel, a filter of a fuel circulation system is choked up by resulting corrosion products. The corrosion sometimes causes generation of through holes. In this sense, the material shall be good of corrosion-resistance.
A steel sheet hot-dip coated with a Pb-Sn alloy (as disclosed in JP57-6133B) and a Zn-coated steel sheet (JP53-19981B) have been used so far, in order to satisfy such requisitions for a material of a fuel tank.
Although the steel sheet hot-dip coated with a Pb-Sn alloy exhibits excellent corrosion-resistance against a fuel solely composed of gasoline, it is likely corroded by alcohol in the case where a fuel tank receives an alcoholic fuel such as methanol or ethanol or a fuel mixture of alcohol with gasoline. When a fuel tank made from the Zn-coated steel sheet is used for receiving degraded gasoline as a fuel or left in a humid environment at a high temperature over a long term, it is attacked by corrosive substances such as formic acid and acetic acid generated by oxidation and degradation of the gasoline, resulting in generation of white rusts.
A steel sheet coated with Al or an Al-Si alloy (as disclosed in JP4-68399B) or an Al-coated steel sheet to which an organic resin film dispersing metal powder therein is applied (hereinafter referred to as “an anti-corrosion painted steel sheet”, as disclosed in JP6-306637A; JP9-53166A) is proposed as a material for a fuel tank in order to eliminate the above-mentioned problems. Corrosion-resistance of the Al-coated steel sheet against an organic acid is enhanced by protective function of an oxide film formed on a surface of the Al plating layer. The anti-corrosion painted steel sheet exhibits good corrosion-resistance against gasoline due to the organic resin film, and metal powder dispersed in the organic film bestows the organic film with electric conductivity so as to facilitate resistance-welding the upper half 2 with the lower half 3.
However, an Al plating layer formed on the steel sheet is inferior of anti-scratching property during press-working, so that substrate steel is often partially exposed to the outside when it is press-worked to upper and lower halves 2, 3. The exposed part will be an origin of internal corrosion, when it is subjected to degraded gasoline including organic acids. A paint film applied to the external surface of the fuel tank does not fulfill its corrosion-preventing function during progress of under-coat corrosion in some cases. On the other hand, the anti-corrosion painted steel sheet is often materially ruptured due to poor lubricity, when it is press-worked to members of a fuel tank.
The plastic reformation of the anti-corrosion painted steel sheet during press-working is improved by applying a lubricate film of an organic resin good of lubricity, anti-corrosion and plasticity to a surface of the anti-corrosion painted steel sheet (as disclosed JP6-306637A, JP9-53166A). However, when the anti-corrosion painted steel sheet after application of the lubricate film is worked to a shape of the fuel tank, the organic resin film as well as the lubricate film are thermally decomposed during resistance-welding. Offensive odor and smoke are abundantly generated by the decomposition, so as to deteriorate the working environment.
Application of an alkali-soluble lubricate film is also proposed, in order to facilitate removal of the lubricate film before welding the upper half 2 to the lower half 3. Although the alkali-soluble lubricate film can be washed off by an alkali, an organic resin film remains on a surface of the steel sheet and causes generation of offensive odor and smoke. Consequently, it is necessary to completely remove the organic resin film from the flanges 5 of the upper and lower halves 2, 3 before welding. Although the organic resin film is peeled off by a chemical method such as application of a proper peeling agent or a mechanical method such as polishing, anyway also needs time and labor.
Lubricity of a resin film may be improved by dispersion of a powdery synthetic resin such as polyolefin or fluororesin to an organic resin film instead of a lubricate film. However, such additive deteriorates adhesiveness and durability of an anti-corrosion paint film applied to the external surface of the fuel tank and causes exfoliation of the paint film during use.